1. Field of the Invention
The present invention relates to a self-excited, one-transistor inverter, and more particularly to an inverter for use in a discharge lighting device that provides high-frequency lighting of a discharge lamp such as a fluorescent lamp.
2. Description of the Related Art
Various types of inverters are known as devices for converting DC power to AC power. Among these is a self-excited, one-transistor, parallel resonant inverter. This inverter comprises a DC voltage source, a transistor used as a switching element, and a parallel resonant circuit. The transistor is turned on and off at a frequency as high as 20 to 100 KHz, thus applying the high-frequency voltage to the resonant circuit. The parallel resonant circuit generates an AC voltage in response to the high-frequency voltage. The AC voltage, thus generated, is applied to a load, i.e., a discharge lamp such as a fluorescent lamp, whereby the lamp is lighted.
In order to control the lighting of the lamp, to compensate for the temperature of the lamp or the inverter, to achieve a smooth start of the lighting, and to protect the lamp after a long use, it is necessary to control the output voltage of the inverter. To control the output voltage of the conventional inverter, two methods are used. The first method is to connect a capacitor to the base of the transistor and control the capacitance of this capacitor by means of a switch. The second method is to connect a variable resistor to the base of the transistor and change the resistance of this variable resistor.
When the first method is used, the output voltage of the inverter cannot be continuously changed since that kind of switch only allows a discrete change in voltage. When the second method is employed, the output voltage of the inverter cannot be controlled over a sufficiently broad range, though it can be varied continuously.